It is known that magnetic sensors can be used to detect the proximity of a ferrous object by detecting the change that the object makes in the flux field produced by a fixed magnet. In one such device, for example as disclosed in U.S. Pat. No. 4,970,463 issued on Nov. 13, 1990 to Wolf et al., a flux sensor is attached to the lateral surface of a permanent magnet at a point midway between the pole ends of the magnet. At this central position the lines of magnetic flux combine to form a zero or null field at the sensing plane of the flux sensor. When a ferrous object is moved toward an end of the magnet, the lines of flux shift toward the ferrous object and therefore the null point of the field is shifted away from the sensing plane and flux passes through the plane. The flux causes the sensor to generate an electric signal that indicates the shift of the null point of the permanent magnetic field and therefore the presence of the ferrous object.
In manufacturing some magnetic proximity sensors, it is necessary to align the null point of the magnetic field of the permanent magnet. It has been found very difficult to consistently and cheaply manufacture large volumes of such detectors because there is a natural variation in the magnetic fields of permanent magnets which affects the alignment of the null point. The physical position of such magnets has therefore been adjusted in manufacturing to set the null point. This process is time consuming and relatively expensive. Also, the magnet can be moved and therefore misaligned if it is potted with other components of the proximity detector.
The manufacturing alignment problem has been resolved for some devices by tuning the magnetic field of the permanent magnet and thereby adjusting the position of the null point. For example, a tuning process is disclosed in U.S. application Ser. No. 08/473,225 of Ramsden, filed Jun. 7, 1995, and entitled Ferrous Article Proximity Detector with Differential Magnetic Sensor Element. This application is owned by the owner of the present invention and is incorporated herein by reference.
It is known that a proximity detector can be made with a single magnet and a Hall-effect sensor that is disposed at either the middle or the north or south poles of the magnet. While such detectors have the advantage of simplicity, they lack some sensitivity as the result of using only one magnet. Moreover, if a magnet tuning process is used in manufacturing, it would be necessary to use, for example, AlNiCo magnets that can be fairly easily tuned. Stronger magnets, for example rare earth magnets, would not necessarily be acceptable for use in the tuning process since they are not easily tuned, even though such magnets could be advantageously used in a proximity detector.
With these issues and considerations in mind, it is an object of the invention to provide a relatively simple, inexpensive and easily manufactured proximity sensor. It is a further object of the invention to provide such a sensor that utilizes interacting fields of two or more magnets to define a null flux point at the sensing plane of a flux sensor. It is another object of the invention to provide a flux sensor that uses at least two different kinds of magnets, one magnet having a high density flux field and the other magnet having a lower density field that can be easily tuned in the manufacturing process. Another object of the invention is to provide simple and relatively inexpensive proximity detectors that are very sensitive to flux changes caused by a ferrous object.
These and other objects of the invention will become apparent from a review of the following.